1. Field of the Invention
This invention relates to contour measuring instruments, and particularly to a contour measuring instrument of the type which takes measurement of the outer surface, i.e., the contour of an object to be measured in a predetermined cross section by tracing the outer surfaces of the object to be measured with a stylus secured to one end of a rotatable arm.
2. Description of the Prior Art
In general, in a contour measuring instrument, a stylus to be brought into contact with the outer surface of an object to be measured is secured to one end of an arm, the other end portion of this arm is rotatably supported by an arm support member, and the center of gravity of the arm is located so that the end that supports the stylus is slightly heavier, whereby the stylus constantly rests in abutting contact with the outer surface of the object to be measured. In measuring the contour of the outer surface of the object by use of a contour measuring instrument of this type, the arm support member is moved in the axial direction of the arm, the stylus is displaced in accordance with the contour of the outer surface of the object to be measured due to the movement of this arm support member so as to turn the arm about its fulcrum where the arm is pivoted, the rotation of the arm is detected by use of contour signal detecting means such as a differential transformer, and the signal detected by this contour signal detecting means is combined with a signal corresponding to the feeding of the arm support member, to carry out the measurement of the contour of the object.
With the conventional contour measuring instrument of the type described, when the measurement is initiated or completed the arm is forced to rotate by a driving mechanism so that the stylus secured to one end of the arm can be comparatively greatly separated from the measured surface of the object to be measured.
However, when measurement is taken on the inner diameter of a cylindrical object or of a hole formed in an object, if the arm is indiscriminately turned to a great degree as described above, there occurs the disadvantage that the stylus or the forward end of the arm comes into abutting contact with the inner wall or the like of the object to be measured.